The ability to reliably and accurately position drops ejected from fluid ejectors, for example, inkjet printheads, at predetermined locations is a critical systems requirement for the printing of high-quality pictorial images and text. Accurate positioning of drops on the receiver is difficult because ejected drops suffer from both stochastic (random) placement inaccuracies and repeating (semi-permanent) placement inaccuracies. Examples of a stochastic (random) placement inaccuracy includes drop-to-drop variations in the contact point of the drop tail as is leaves the ejector surface and fluctuations in the airflow around the printhead. Examples of repeating (semi-permanent) placement inaccuracies include permanently malformed ejectors and particulate debris contacting the ejector nozzle plate.
In some situations, accurate positioning of drops may be achieved by locating the receiver in close proximity to the printhead, so that drops which are angularly misdirected do not have time to travel too far from their desired location on the receiver in the plane of the receiver. However, overly close spacing may cause mechanical contact between the printhead and the receiver possibly resulting in printhead damage.
Other strategies to control drop locations include the use of airflow or electric fields oriented in the direction of the drop trajectories to guide drops to desired locations as well as the application of electric fields perpendicular to the direction of the drop trajectories to guide drops to desired locations. However, these strategies need to use very large airflows or very high electric fields to influence drop trajectories which possibly resulting in image artifacts and reduced system reliability.
Accurate positioning of drops on the receiver is also limited by the formation of satellite drops during drop breakup or by drop recombination as drops travel along their trajectories. Drops of unusually small or large sizes are produced which reduce image quality or cause reliability problems due to fluid accumulation at unwanted regions. Although satellite formation can be controlled to some extent by ink formulation or printhead operation parameters, these solutions typically reduce image quality or printer performance, for example by requiring special ink formulations not optimized for image quality or by necessitation reduced printing speeds.
The inverse relationship between frequency of operation and drop control also contributes to accurately positioning drops. In general, it is desirable to operate inkjet printers at the highest possible frequencies for reasons of productivity. However, drop placement typically suffers at high frequency operation while the propensity of satellite formation or drop recombination typically increases.